WARNING: Make sure you use exactly the same Collator at
index and query time -- CollationKeys are only comparable when produced by
the same Collator. Since RuleBasedCollators are not
independently versioned, it is unsafe to search against stored
CollationKeys unless the following are exactly the same (best practice is
to store this information with the index and check that they remain the
same at query time):

The ICUCollationKeyAnalyzer in the analysis-icu package
uses ICU4J's Collator, which makes its
its version available, thus allowing collation to be versioned
independently from the JVM. ICUCollationKeyAnalyzer is also significantly
faster and generates significantly shorter keys than CollationKeyAnalyzer.
See http://site.icu-project.org/charts/collation-icu4j-sun for key
generation timing and key length comparisons between ICU4J and
java.text.Collator over several languages.

CollationKeys generated by java.text.Collators are not compatible
with those those generated by ICU Collators. Specifically, if you use
CollationKeyAnalyzer to generate index terms, do not use
ICUCollationKeyAnalyzer on the query side, or vice versa.

You must specify the required Version
compatibility when creating CollationKeyAnalyzer: